Some optical systems for print acquisition use the principle of frustrated total internal reflection in order to produce images of very high contrast. The finger (or other object to analyze) is then place on a prism or slide with an inclined face. The print acquisition system is furthermore provided with an illumination means and with an imaging system. Two arrangements are possible: bright-field and dark-field.
Other types of sensor have been developed: sensors with “face-on” imaging, such as that disclosed by European Patent Application No. EP1789908; contactless sensors such as those described in French Patent Application No. FR2927713 or PCT Pub. No. WO201490750, and flat contact sensors (TFT).
Whatever the technique used, spoofing means have been developed, among which is the use of finger moldings (or moldings of the object to analyze). These moldings may be made from various materials, which are generally available in all DIY stores or hobby stores, such as latex or silicone. It is also known, as a spoofing means, to use a print-out of the print on a sheet of paper.
The purpose of the acquisition of a print is to compare the acquired print with one or more images stored in a database and, in case of match, to authenticate or identify the carrier. This serves in particular to give a right such as an access right, a signature right, or a right to use a system, etc. Spoof detection makes it possible, automatically, not to identify or authenticate a person who presents a replica to the sensor.
To fulfill this function, several techniques are possible.
PCT Pub. No. WO2006/082550 proposes to use the acquisition of a fluorescence spectrum, either to verify the presence in the object analyzed of one or more fluorophores that are native, that is to say which are naturally present in the human body (collagen tryptophan, etc.) if is solely a matter of eliminating the risk of being in the presence of a synthetic replica, or to verify the match between the acquired fluorescence spectrum and a reference fluorescence spectrum if it is a matter of authenticating a person whose identity is presupposed. The technique proposed by PCT Pub. No. WO2006/082550 requires the use of a spectrometer that is sufficiently energy resolving, which is an additional burden on the manufacturing cost of the print acquiring system. Furthermore, the spectrometry analysis increases the processing time for the print, which may generate an undesirable queue.
The use of native fluorescence to detect a living finger in the context of print measurement is also disclosed by U.S. Patent Pub. No. 20140037158, which describes a system measuring the fluorescence of sweat induced by an electromagnetic field. In this patent, the sensor causes an electric current to pass in the finger to stimulate the production of sweat and localize the position of the pores by fluorescence. This patent assumes that the position of the pores, located by fluorescence induced by an electromagnetic field, may be considered as biometric data enabling a subject to be authenticated in relation to the presupposed identity.
The invention disclosed by U.S. Patent Pub. No. 20140037158 has in particular the following drawbacks:                the imaging of the pores and locating them require a high-resolution sensor, which burdens the cost of the final system;        the process of inducing the sweat requires more than a second to work, which is detrimental to the ergonomics of the device. As a matter of fact, most print sensors have to operate in less than a second to avoid queues at the locations where they are installed.        